Basement Control on Volcanic Edifice Stability: Application to Mount Etna

Instability in volcanic edifices is consequence of several factors, e.g. growth and load of the edifice, magmatic intrusions and related processes, such as stress field changes, fluid circulation, earthquakes. Nonetheless, the presence of a sedimentary basement is a key control factor for instability and Mount Etna is falling into this category: it is a stratovolcano lying on top of a sedimentary basement and presenting flank instability in its eastern-southern sector. It is a well-monitored volcano because of its threat on the surrounding cities, and the wealth of multidisciplinary data collected so far helps to better constrain the dynamic of the flank. However, the deformation mechanisms are still matter of debates. Many factors can contribute to lowering the mechanical resistance of the basement, and sedimentary rocks are particularly sensitive to: a) presence of water, b) pressure, c) temperature. In this thesis, the experimental approach aims to characterize the mechanical strength of the Etnean rocks, monitoring the evolution of their behaviour from fracture nucleation to sliding under relevant conditions of P, T and pore fluid pressure, mimicking natural conditions. The study focuses on the influence of a thermal gradient induced by dyke intrusions into the host rock. Further, the development of a new piece of equipment for triaxial apparatus allows collecting additional data on the frictional resistance of Etnean gouges. Results demonstrate that rocks in the basement are transitioning from brittle to ductile behaviour in relative shallow conditions (i.e. 1-2 km), and mixtures of synthetic gouges show stable sliding with the friction coefficient sensitive to clay content. These results can help to explain the particular seismic-aseismic behaviour of the easternmost fault system bordering the sliding flank (e.g. Pernicana Fault System). Further, this experimental approach is well applicable to other volcanoes showing similar conditions.